Grinding method



United States Patent 3,313,492 GRINDING METHOD Daniel A. Jacobs andJames B. Duke, Metuchen, N.J., assignors to Minerals & Chemicals PhilippCorporation, Woodbridge, N.J., a corporation of Maryland N0 Drawing.Filed Dec. 24, 1964, Ser. No. 421,188 17 Claims. (Cl. 241-21) Thisinvention relates generally to the grinding of finely divided solidmaterial and is directed specifically to the grinding of finely dividedsolid material which ofiers resistance to being ground to an irnpalpablepowder.

Finely divided material, especially mineral matter, is usuallymicronized in mills having high energy input. In one type of micronizer,usually referred to as a fluid energy mill, the material to be ground issuspended in a stream of a gaseous material, such as air or steam.Subdivision of the solid results from high speed particleto-particleimpact. In another type of micronizer, which also utilizes a very highenergy input, the material is suspended in a liquid along with solidgrinding media and the mixture is subjected to intensive churning oragitation. Various forms of the latter type of micronizer have beensuggested, one example being the process described in a publication ofthe US. Department of the Interior, Bureau of Mines, by I. L. Feld etal., entitled, Paper-Coating Clay From Coarse Georgia Kaolin by a NewAttrition Grinding Process.

In contrast to the aforementioned processes, which require high energyinput and intensive agitation of the charge material, is the grindingprocess described in U.S. 3,097,801 to James B. Duke, entitled, Methodfor Comminuting Kaolin Clay. This process constitutes a fundamentaldeparture from the prior art micronizing processes in that it utilizesvery mild agitation with relatively low energy input. In accordance withthis process, finely divided clay material is formed into a fiuidaqueous slip and mixed with sand that is fine enough to pass through amesh screen and is coarse enough to be retained on a 35 mesh screen(i.e., minus 10 plus 35 mesh sand). A smooth cylinder is partiallyfilled with the mixture and is rotated at a relatively low peripheralspeed, e.g., 200 to 300 feet per minute. The results have beenremarkable in that the micron size clay platelets, which are very hardand resistant to grinding, have been eliectively comminuted.

An object of this invention is to improve the efiiciency of theaforementioned grinding process in which a fluid slip of material to beground and finely divided grinding medium is rotated at relatively lowspeed in a drum or cylinder.

A specific object is to improve upon the rate of grinding obtainable byrotating a slip of finely divided charge material with particulategrinding medium.

This invention is a result of the unexpected discovery that by employinga finely divided grinding medium of substantially greater specificgravity than the sand grinding medium in the aforementioned process ofJ. B. Duke, and utilizing the heavy grinding medium in the form of roundparticles, the rate of grinding that can be realized :by controlledrotation of a slip of the charge material with the grinding medium isincreased to a remarkable extent. This result was surprising since itwas expected that a finely divided, high density grinding medium wouldhave too much inertia to provide the required grinding action. This wasespecially true because a general characteristic of grinding processesusing fine grained grinding media (e.g., the processes described in U.S.1,956,293 to P. Klein et al., and U8. 2,581,414 to Hochberg) is thatthese processes utilize lightweight grinding media, such as sand orceramics. These processes do not utilize heavy metals such as steel,which are generally restricted to use in processes employing grindingmedium in the form of large particles, e.g., the Az-inch balls used in aball mill. The latter mills are used for making relatively coarse grindsand function in a different manner from any of the aforementionedm-icronizing systems which grind in the minus 10 micron size range.Furthermore, it was very surprising that the heavy grinding medium waseffective when it was in the form of round particles and wascomparatively ineffectual when in the form of angular particles.

In carrying out the present invention, the grinding medium that is usedis an abrasive or burnishing material that has a specific gravitysubstantially greater than, preferably at least double, the specificgravity of sand (silica), which is about 2.6. A preferred grindingmedium is steel, which has a specific gravity of about 7.0 to 7.9,depending upon the composition. An essential characteristic of the heavyfinely divided grinding medium is that it is generally round in form.Thus, we can use steel grinding medium in the form of small steel ballsabout -inch to /a-inch in diameter (about 6 to 20 mesh, Tyler). Thedesired results, however, are not obtained with an angular heavygrinding medium, such as steel grit or steel shavings. The abrasivemedium does not need to be truly spherical in form since steel shot,which is generally ovoid in shape, has produced excellent results. Othergrinding media having high specific gravity include: ferrosilicon,especially 15% Fe ferrosilicon which has a gravity of 6.8; tungstencarbide, a very heavy material having a specific gravity of 15.6; andmagnetite which has a specific gravity of 5.2. Another essentialcharacteristic of the grinding media is that it is composed of particlespredominantly within a specific mesh size range; namely, minus 4 plus 35mesh (Tyler). In other words, the major weight percentage, andpreferably substantially all, of the grinding media should be fineenough to pass a 4 mesh (Tyler) screen and be coarse enough to beretained on a 35 mesh (Tyler) screen. The desired results are notobtained with grinding media that are too fine' or too coarse.

A wide variety of minus 325 mesh (about 40 microns) material, naturaland synthetic, can be reduced in particle size by the process or" thisinvention. The process works well with crystalline material that is veryhard and difiicult to grind in the minus 10 micron size range, e.g.,kaolin clay, calcite and tale. The process is also very effective withnon-crystalline (amorphous) material, such as, for example, calcinedkaolin clay. The process is especially amenable to comminuting materialthat is relatively low in specific gravity, i.e., material which has aspecific gravity less than about 3.0. Excellent results have beenobtained in grinding material having a specific gravity within the rangeof about 2 to about 3. Thus, a characteristic of the process is that thedensity of the material to be ground is appreciably less than thedensity of the grinding medium that is used.

In putting the invention into practice, the minus 325 mesh grindablematerial is slipped with water, or other liquid, using a dispersingagent, if necessary, to impart fluidity to the slip. With most grindablematerial, the proportion of solid grindable material to liquid will bewithin the range of about 10 to 50 parts by weight of the solidgrindable material to 50 to parts 'by weight of liquid, i.e., the chargeof material to be ground is in the form of a slip containing 10% to 50%solids, weight basis, exclusive of the grinding medium. The solids ofthe slip Will vary with the nature of the liquid and solid and islimited by the necessity for forming a slip which is distinctly fluid.In the case of clay or calcined clay,

solids slips are preferred.

The slip is charged to a horizontal drum that is free from flights,bafiles or agitators, with the slip only partially filling the drum. Itis with a volume of slip such that the slip with added grindpreferableto charge the drum ing medium fills about half the drum or is somewhatbelow the midpoint of the drum when the drum is at rest.

The finely divided grinding medium is added to the fluid slip (or viceversa) in amount such that the particles of grinding medium arecompletely immersed in the slip when the drum with slip and grindingmedium is at rest. Especially good results have been realized when thevolume of the particles of grinding medium was about onethird the volumecapacity of the grinding drum and the entire contents of the drum,including the grinding medium, occupied about half the volume of thedrum under quiescent conditions. In other words, the particles of steelgrinding medium occupied about two-thirds of the total volume of thecharge to the drum.

The horizontal drum and contents is rotated about the horizontal axis ofthe drum at a specific speed. This speed is such that the grindingmedium and material to be ground remain suspended in the slip during thegrinding and the slip with suspended medium tends to rise in the drum inthe direction of rotation of the drum. The slip with suspended grindingmedium remains substantially as a unitary or integral body (as opposedto a strongly agitated body in which extensive splashing of liquidoccurs or in which a substantial part of the grinding medium rises outof the liquid and then descends back into the liquid). Within theunitary body, the grinding medium moves in a generally continuous,substantially elliptical closed path.

The slip containing grinding medium is tumbled until the micron-sizeparticles in the slip have been comminuted to the desired size. The timeto effect the desired degree of comminution is typically within therange of minutes to 10 hours, and is usually within the range of /2 hourto 2 hours.

After tumbling is completed, the slip is separated from the grindingmedium by sedimentation and decantation, or by screening. To obtain thesolids in the slip in dry form, the slip can be fiocced, filtered, driedand ground. Employing a metallic grinding medium such as steel, somemetal-staining of the charge material may result from the continuousimpact of the mineral charge with the metallic medium. If necessary ordesirable, the metal stain, such as iron-stain, can be removed from themineral particles by means of bleaching reagents such as, for example,zinc hydrosulfite.

While the suspension in the grinding drum consists of minus 325 meshmaterial to be ground, liquid (usually water) and finely dividedgrinding medium, it will be distinctly understood that other materialcan be present in the suspension. For example, a dispersing agent can beincorporated into the mixture to obtain suspensions of adequate fluidityor mobility. Small quantities of alkali or chelating agents can be addedto the suspensions to prevent or minimize metal-staining. Also, mixturesof high specific gravity grinding medium can be used.

In putting this invention into practice, the grinding mill can be anyhorizontal vessel having a smooth cylindrical interior, such as a drum.The interior surfaces of the mill must be smooth and wettable by theliquid medium. The desired results are not realized when the liquid inthe charge does not wet the inner surface of the drum. As mentionedhereinabove, the drum must be free from baffles or other agitator means.The grinding vessel is provided with means for continuously rotating thevessel about its horizontal axis, as for example, by rollers on whichthe vessel rests. For small scale experimental runs, an open-neck glassjar having a cylindrical body will suflice. Commercial mills may beadapted for closed circuit grinding 'by connecting the mill in serieswith a classification system to remove fines as they are formed and torecycle the oversize.

The following examples are given to illustrate further the presentinvention and to show some of its advantages.

In these examples, unless otherwise indicated, all proportions representweight proportions and all mesh sizes refer to values measured on Tylerscreens. All particle size of charge material refers to the size of theultimate particles and is reported as microns (e.s.d.). Particle size inthe micron size range was determined by the sedientation proceduredescribed in TAPPI Standards, T649 SM54; particle size data in rangesbelow 0.5 micron were determined by a simple modification of the TAPPImethod which provided for the use of a long arm centrifuge, as describedin a publication by F. H. Norton and S. Speil in I. Am. Ceramic Soc.,21, 89 (1938).

The onegallon experimental rotating mill used in the examples was anopen mouth cylindrical glass jar resting horizontally on a pair ofhorizontal rollers connected to a variable speed drive was to rotatecontinuously the jar about its horizontal axis without vibrating thejar. The speed of 100 r.p.m. used in the experiments corresponds to aperipheral velocity of about 300 ft./ min. and was such that thecontents of the jar had the required action for grinding, as describedhereinabove.

Example 1 Calcined kaolin clay was subjected to various grinding teststo obtain a minus 2.0 micron product.

In accordance with this invention, an aqueous slip of the minus 325 meshcalcined kaolin clay was tumbled in a rotating drum with steel balls(about 20 mesh). The results were then compared to the results obtainedwhen an equal volume of minus 10, plus 35 mesh angular silica sand (suchas was used in illustrative examples in U.S. 3,097,801 to James B. Duke)was substituted for the steel balls.

The starting clay used in the tests was obtained by calcining a finesize fraction of Georgia kaolin clay in a Nichols Herreshofi furnace.The fine size fraction of clay was 100% minus 325 mesh and about 90%minus 2.0 microns before calcination and about 90% minus 3.5 microns andabout 75% minus 2.0 microns after calcination. The average particle sizeof the calcined clay feed was 1.3 microns e.s.d. (The term averageparticle size is described in U.S. 3,097,801.)

The calcined clay was slipped and then comminuted in the one-gallon jaras follows. Two hundred grams of the clay was added to the jar togetherwith 600 ml. of water, 0.7 gram of tetrasodium pyrophosphate and 1400ml. of the & steel balls (Abbott). The jar with contents was placedhorizontally on a pair of horizontal rollers connected to a variablespeed drive and the rollers were rotated in opposite directions at thesame speed. As a result, the jar was continuously rotated about itshorizontal axis. The charge was rotated at about 98 rpm. (300 ft./min.)for about two hours. After tumbling, the slip was decanted from thesteel balls and then screened on a 325 mesh screen to separate the slipfrom the steel balls. The slip was flocced by addition of sulfuric acid,filtered and dried. A particle size analysis of the calcined clayproduct was then made. The test was then repeated with an equivalentvolume of the sand (Whitehead #1) substituted for the steel balls. Inboth cases, a two-hour grinding period was used.

A particle size distribution curve of the minus 325 mesh portion of thefeed was obtained from the particle size analysis of the feed and thiscurve was compared to curves for the products obtained by tumbling thecalcined clay with the steel balls or sand for the same grindingperiods. The results, summarized in Table I, show that using the smallsteel balls, in accordance with this invention, the average particlesize of the calcined clay was reduced to a value of only 0.58 micron.This represented a 50% reduction in average particle size since the feedhad an average particle size of 1.3 microns. Using the sand grindingmedium of the prior art, however, the average size of the groundmaterial was 0.78 micron. Thus, when the heavy steel grinding medium wasemployed, the average particle size of the ground product was 40% lessthan when the sand grinding medium of the prior art was used.

Data in Table I also indicate that the steel balls were especiallyeffective in grinding the finer size fractions of the feed, particularlythe minus 1.0 micron fraction. Thus, using the steel balls, there wasabout twice as much new minus 1.0 micron calcined clay produced bygrinding as compared to sand grinding.

TABLE I.EFFECT OF GRINDING MEDIA ON GRINDING CALCINED CLAY A series ofgrinding tests, similar to those of Example I, was carried out with acoarse size fraction of uncalcined Georgia kaolin clay (NoKarb) that hadbeen obtained by hydraulic classification of a Georgia kaolin claycrude. The average particle size of the feed clay was 4.8 microns andthe objective of the grinding operation was to obtain a maximum contentof minus 2.0 micron clay particles in the product. In the tests, theminus plus 35 mesh sand was compared to the steel balls in grinding runscarried out for various grinding periods. It was found that the 3& steelballs produced about the same size product in 30 minutes as an equalvolume of the sand produced in about 120 minutes. By this comparison,the steel balls were about four times as effective as the sand inmicronizing the coarse size fraction of naturally-occurring clay.

Example 111 Tests were made to illustrate the use of steel grindingmedium in the form of steel shot to grind the NoKarb clay with maximumproduction of minus 2.0 micron clay particles. As in the previousexamples, the results of the grinding with heavy grinding media werecompared to results with the Whitehead #1 silica sand.

Two different samples or steel shot, each minus 10 plus 35 mesh, wereused. The shot samples, which were supplied under the trade names S280and S390 (Wheelabrator Corporation), had been produced commercially bymelting premium steel scrape, shotting, heat reating and sizing. Thedensity of the steel shot was reported as being a minimum of 7.3 g./cc.,as determined by the displacement of alcohol. Screen analyses of the twoshot samples are given below, along with a screen analysis of theWhitehead #1 sand. These analyses show that all grinding media was minus10 plus 35 mesh and, therefore, generally similar in size. The analysesshow also that the S280 shot was somewhat finer than the sand, while theS390 shot was somewhat coarser than the #1 sand.

SCREEN ANALYSES OF GRINDING MEDIA Weight Percent Mesh Size Steel Shot #1Sand Composite 100. 0 100. 0 100.0

1 Trace.

The NoKarb clay was slipped in water, using 200 grams clay and 600 gramswater. The slip was placed in the oneagallon glass jar together with1400 ml. of grinding medium and 0.3 gram 0 brand sodium silicate(containing about 9% Na O, 29% SiO and 62% water, weight basis). The jarwas continuously rolled at r.'p.m. for 30 minutes. The grinding mediumwas then separated from the clay slip by screening through a 325 meshscreen. The slip was flocced with sulfuric acid, filtered, dried andanalyzed for particle size distribution by the sedimentation method. Theaverage particle size of products obtained with the different minus 10plus 35 mesh grinding media is tabulated in Table II.

TABLE II.-EFFECT OF COMPOSITION OF GRINDING MEDIA ON THE GRINDING OF ACOARSE SIZE FRAC- TION OF KAOLIN CLAY [4.8 microns average particlesize] Average particle size of Grinding media: ground clay product,microns Steel Shot (S280) 1.40 Steel Shot 5390 1.40 #1 Sand 2.74

Data in Table II show that when minus 10 plus 35 mesh steel shot wasused, the average particle of ground clay was twice as small as whensand of about the same size as the shot was used. A comparison of theaverage size of ground clay products with the average size of the clayfeed (4.8 microns) shows that the steel shot was about 200% moreeffective than sand in reducing the average particle size of the clay.

Using the sand, it was found that about 37% of the product was finerthan 2.0 microns, as compared to 16% for the feed. When the S280 shotwas employed, 65% of the product was minus 2.0 microns. Thus, more thantwice as much of the desired new minus 2.0 micron clay was obtained withthe steel shot.

Example IV To study the effect of shape of a steel grinding medium ongrinding efficiency in the process, samples of commercial steel gritsimilar in particle size distribution to the S280 and S390 steel shotwere obtained. The grit (G12 and G16, products of WheelabratorCorporation) had been produced by crushing steel shot and screening thecrushed material. The density of the steel grit samples was reported asbeing a minimum of 7.6 grams per cc., as determined by the displacementof alcohol. The grit was generally in the form of quartered balls andwas observed to be distinctly angular in shape when examined under amagnifying glass.

Following are screen analyses of the minus 10 plus 35 mesh grit samples,showing that the grit was generally similar in size to the steel shotgrinding medium of the previous sample. One grit sample (G12) was alittle coarser than the sand grinding medium of the prior art and theother sample of lgrit (G16) was a little finer than the sand grindingmedium.

SCREEN ANALYSIS OF GRINDING MEDIA Weight Percent l lesh Size Steel GritComposlte. 100.0 100.0

Using the steel grit samples as the grinding medium, the procedure ofthe preceding example was repeated in full. It was found that when eachof the angular grits was used in place of the substantially round shot,the average particle size of the products was 2.4 microns, in comparisonto the 1.4 average particle size obtained using shot of similar size.Using the G12 or G16 grit, only about 40% by weight of the products wasfiner than 2.0 microns, as compared to 16% minus 2.0 micron clay in thefeed and 65% minus 2.0 micron clay roduct using the shot (S280). Thus,about 25% more of the desired minus 2.0 micron clay was produced usingthe steel shot instead of grit of similar size.

Example V In accordance with this invention, minus 325 mesh limestonehaving a density of 2.71 g./ cc. (Chemcarb #11) was micronized with the/s steel balls by placing 200 grams of the limestone, 600 ml. soft waterand 6965 grams of the A3 steel balls in the one-gallon jar and rotatingthe jar with contents for 2 hours at about 100 r.p.m. The procedure wasrepeated substituting an equal volume of the Whitehead #1 sand for the/s" steel balls.

Following are particle size distribution of the limestone charge andground products, illustrating the marked superiority of the small steelballs over the sand in micronizing the limestone.

TABLE III.EFFECT OF COMPOSITION OF GRINDING MEDIA ON THE GRINDING OFLIMESTONE A sample of talc ore was tabled and a minus 35 mesh aqueoustable concenrate was obtained. In accordance with this invention, 426grams of the aqueous table concentrate at 47.0% solids was diluted to25% solids with soft water and placed in the one-gallon jar with 6965grams of the /s" steel balls. The jar with contents was rolled for 2hours at 100 r.p.m. and the contents screened over a 14 mesh screen toseparate the grinding medium from the talc pulp. The minus 14 mesh talcpulp was thickened by sedimentation and bleached with zinc hydrosulfite,filtered, washed and dried at 175 F. In order to determine the size ofindividual talc particles rather than the size of talc agglomerates inthe dried product, the aqueous pulp was screened by means of a wetscreening process through a 325 mesh screen. It was found that when theore pulp was ground in the manner described, the product contained 92.6%by weight of material that would pass the 325 mesh screen.

In contrast, when a similar sample of tabled talc ore concentrate at 50%solids was ground for 2 hours in a type A Abbe Single Assay Mill (8.75dia.) containing 12.7 pounds of Alunclum cylinders and rolled at r.p.m.for 4 hours, screened, filtered and dried, the product contained only64.1% of minus 325 mesh ore when tested by the wet screening procedure.

The specific gravity values mentioned herein refer to values obtained byassigning the density of Water at 4 C. and normal atmospheric pressurethe value of unity.

We claim:

1. A process for reducing the size of micron-sized particles whichcomprises:

partially filling a bathe-free, agitator-free, cylindrical drum having asmooth inner surface with a fluid suspension comprising saidmicron-sized particles, liquid and particulate grinding medium having adensity appreciably greater than the density of said particles and beingcomposed largely of substantially round particles fine enough to passthrough a 4 mesh sieve and coarse enough to be retained on a 35 meshsieve,

said liquid being employed in amount suflicient to form a fluid slipwith said micron-sized particles and said particulate grinding mediumbeing present in amount such that the particles thereof are distributedthroughout a substantial amount of the total volume of said fluidsuspension,

and continuously rotating said drum about its horizontal axis at a speedless than critical and such that said suspension within the drum is inthe form of an in= tegral body occupying substantially only the lowerportion of the drum, the inividual particles of grinding medium withinthe suspension traveling in continuous, generally elliptical pathswithin the suspension, said rotation being continued until saidmicronsized particles are reduced in size.

2. The method of claim 1 in which the grinding medium has a density atleast twice as great as the density of the micron-sized material to beground.

3. The method of claim 1 in which the grinding medium is steel.

4. The method of claim 1 in which the grinding medium is minus 10 plus35 mesh steel shot.

5. The method of claim 1 in which the micron-sized material to be groundhas a specific gravity within the range of about 2 to about 3 and thegrinding medium has a specific gravity within the range of about 6 toabout 8.

6. A process for reducing the size of micron-sized particles which,comprises:

partially filling a baflle-free, agitator-free cylindrical drum having asmooth inner surface with a fluid suspension comprising saidmicron-sized particles, water and substantially round steel particlesfine enough to pass through a 4 mesh sieve and coarse enough to beretained on a 35 mesh sieve,

said water being employed in amount suflicient to form a fluid slip withsaid micron-sized particles and said steel particles being present inamount such that the particles thereof are covered completely by theliquid in said suspension and occupy a substantial amount of the totalvolume of said fluid suspension,

and continuously rotating said drum about its horizontal axis at a speedless than critical and such that said suspension is in the form of anintegral body occupying substantially only the lower portion of thedrum, the individual particles of steel within the suspension travelingin continuous, generally elliptical paths within the suspension, saidrotation being continued until said micron-sized particles are reducedin size.

7. The method of claim 6 in which said steel particles are in the formof balls.

8. The method of claim 6 in which said steel particles are in the formof shot.

9. The method of claim 6 in which said micron-sized particles comprisenaturally occurring kaolin clay and said steel is in the form of minus10 plus 35 mesh shot.

10. The method of claim 6 in which the steel particles occupy aboutone-third the total volume of the drum.

11. The method of claim 6 in which the fluid suspension occupies aboutonehalf the volume of the drum, and the steel particles occupy abouttwo-thirds of the total volume of said suspension.

12. A method for grinding minus 325 mesh mineral particles whichcomprises charging a cylindrical horizontal battle-free, agitator-freedrum with a fluid suspension comprising minus 325 mesh mineralparticles, Water and minus 10 mesh plus 35 mesh, generally roundparticles of steel shot, said suspension being used in amount to fillabout half of said drum and said steel particles being employed inamount sufiicient to occupy about two-thirds of the total volume of thesuspension, continuously rotating the drum at a speed less than criticalwith suspension for a time Within the range of about 10 minutes to about2 hours, and separating the mineral particles and water from the steelshot.

13. A process for reducing the size of micron-sized particles whichcomprises:

partially filling a baffle-free, agitator-free cylindrical drum having asmooth inner surface with a fluid suspension comprising saidmicron-sized particles, liquid and 1/32" steel balls, said liquid beingemployed in amount sufficient to form a fluid slip with saidmicron-sized particles and said steel balls being present in amount suchthat the particles thereof are distributed throughout a substantialamount of the total volume of said fluid suspension,

and continuously rotating said drum about its horizontal axis at a speedless than critical and such that said suspension within the drum is inthe form of an integral body occupying substantially only the lowerportion of the drum, the individual steel balls within the suspensiontraveling in continuous, generally elliptical paths Within thesuspension.

14. A method for grinding kaolin clay which comprises partially fillinga baffle-free, agitator-free, horizontal,

10 smooth inner-surfaced drum with a fluid aqueous kaolin clay slipcontaining 10% to clay solids and minus 10 mesh plus 35 mesh, generallyround particles of steel shot, said steel shot being present in amountsuch that the particles thereof are distributed throughout a substantialamount of the total volume of said clay slip,

rotating said drum about its horizontal axis at a speed below criticaland such that the steel shot is suspended in said clay slip and saidsuspension is in the form of an integral body occuping substantiallyonly the lower portion of the drum, the individual particles of steelshot within the suspension traveling in continuous, generally ellipticalpaths Within the suspension,

and rotating said drum in said manner until said clay is reduced insize.

15. The method of claim 14 wherein said clay is a calcined clay.

16. A method for grinding kaolin clay which comprises filling abaffle-free, agitator-free, horizontal, smooth innersurfaced drum abouthalf full With a mixture of a fluid aqueous kaolin clay slip containing10% to 50% clay solids and minus 10 mesh plus 35 mesh, generally roundparticles of steel shot, said steel shot being present in amount suchthat the particles thereof occupy about tWothirds of the total volume ofsaid mixture,

rotating the drum with said mixture at a speed less than critical andsuch that said mixture is a suspension in the form of an integral bodyoccupying substantially only the lower portion of the drum, theindividual particles of grinding medium Within the suspension travelingin continuous, generally elliptical paths within the suspension, saiddrum being rotated for a time sufiicient to reduce the particle size ofsaid clay, and separating said steel shot from said clay slip.

17. The method of claim 16 wherein said clay is a calcined clay.

References Cited by the Examiner UNITED STATES PATENTS 2,991,017 7/1961Hukki 241-l84 3,008,656 11/1961 Weston 241-184 3,008,657 11/1961Szegvari 241-184 WILLIAM W. DYER, JR., Primary Examiner.

G. A. DOST, Assistant Examiner.

1. A PROCESS FOR REDUCING THE SIZE OF MICRON-SIZED PARTICLES WHICHCOMPRISES: PARTIALLY FILLING A BAFFLE-FREE, AGITATOR-FREE, CYLINDRICALDRUM HAVING A SMOOTH INNER SURFACE WITH A FLUID SUSPENSION COMPRISINGSAID MICRON-SIZED PARTICLES, LIQUID AND PARTICULATE GRINDING MEDIUMHAVING A DENSITY APPRECIABLY GREATER THAN THE DENSITY OF SAID PARTICLESAND BEING COMPOSED LARGELY OF SUBSTANTIALLY ROUND PARTICLES FINE ENOUGHTO PASS THROUGH A 4 MESH SIEVE AND COARSE ENOUGH TO BE RETAINED ON A 35MESH SIEVE, SAID LIQUID BEING EMPLOYED IN AMOUNT SUFFICIENT TO FORM AFLUID SLIP WITH SAID MICRON-SIZED PARTICLES AND SAID PARTICULATEGRINDING MEDIUM BEING PRESENT IN AMOUNT SUCH THAT THE PARTICLES THEREOFARE DISTRIBUTED THROUGHOUT A SUBSTANTIAL AMOUNT OF THE TOTAL VOLUME OFSAID FLUID SUSPENSION, AND CONTINUOUSLY ROTATING SAID DRUM ABOUT ITSHORIZONTAL AXIS AT A SPEED LESS THAN CRITICAL AND SUCH THAT SAIDSUSPENSION WITHIN THE DRUM IS IN THE FORM OF AN INTEGRAL BODY OCCUPYINGSUBSTANTIALLY ONLY THE LOWER PORTION OF THE DRUM, THE INDIVIDUALPARTICLES OF GRINDING MEDIUM WITHIN THE SUSPENSION TRAVELING INCONTINUOUS, GENERALLY ELLIPTICAL PATHS WITHIN THE SUSPENSION, SAIDROTATION BEING CONTINUED UNTIL SAID MICRONSIZED PARTICLES ARE REDUCED INSIZE.